Speed and Performance in Liquid Chromatography

This chapter is aimed at presenting the basic aspects of both performance – primarily measured using plate count (N) – and speed as measured by the column dead time (t0), that is, the elution time of an analyte that is unretained but fits in all the pores of the packing material. Because one can always improve N by increasing t 0, for example by using a longer column, we consider in some detail the process of optimization of performance measured by t 0 /N. The three methods of optimization, namely one-parameter (van Deemter), two-parameter (Poppe), and three-parameter (Knox-Saleem-Halasz), as well as their limitations, are developed in detail. One of the best metrics for comparing the resolving power of one-and two-dimensional liquid chromatography is the peak capacity (nc) which is the number of peak maxima that can be fit in a given separation space. Because gradient elution LC, except at exceeding short times, generally gives higher peak capacities than isocratic elution, the basics of gradient LC (especially reversed-phase separations) are developed, including the gradient peak compression process. Also included is an appendix of background material covering fundamental concepts that control column dynamics, pressure drop across the column, and the fundamental limitation of chromatography at very high pressures – namely the frictional heating of the mobile phase and its impact on peak dispersion.